1. Field of the Invention
The invention relates to an image sensor, and more particularly, to an image sensor with complementary metal oxide semiconductor (CMOS) pixel, and a method for driving the same
2. Description of the Related Art
An image sensor captures images by using an energy response characteristic of a semiconductor material, to detect the energy (for example, light). The image sensor can be classified generally as either a CMOS image sensor and a charge coupled device (CCD) image sensor. The CMOS image sensor can be operated by a single power voltage source. So, the CMOS image sensor has the advantages of lower power consumption and smaller size, as compared with the CCD image sensor. Therefore, the CMOS image sensor is generally used.
FIG. 1 illustrates a conventional image sensor. The conventional CMOS image sensor has a pixel sensor 101 and a double sampling circuit 103. According to the conventional image sensor, when a reset signal RS goes to a logic HIGH, a reset transistor 101a is turned on and the voltage at a node N101 rises. Then, via a driving transistor 101c and a selecting transistor 101d, reset data, having a voltage of nearly VDD, is transferred to a data input-output (I/O) line DIO And then, when the reset signal RS goes to a logic LOW and a control signal TX goes to a logic HIGH, the reset transistor 101a is turned off and a transfer transistor 101b is turned on. Via the driving transistor 101c and the selecting transistor 101d, signal data in a photo-diode 101e is transferred to the data I/O line DIO. Then, the reset data and the signal data transferred to the data I/O line DIO are sampled by a double sampling circuit 103 connected to the data I/O line DIO.
In the conventional image sensor, the signal data is sampled and outputted after the reset data is sampled and outputted. Therefore, the signal data is mostly maintained in the data I/O line DIO
According to the conventional CMOS image sensor, the double sampling circuit 103 includes a sampling transistor 103a, an assistance capacitor 103b, a coupling capacitor 103c and a pre-charging transistor 103d. The sampling transistor 103a cuts the sampled signal data off from the data I/O line DIO, whereby an output signal VOUT having a stable voltage level can be generated via an output terminal DQ The assistance capacitor 103b prevents a node N103 from floating. If the assistance capacitor 103b is not implemented, the node N103 can be floated when the sampling transistor 103a is turned off. A capacitance, of a virtual capacitor created by the assistance capacitor 103b and the coupling capacitor 103c, is nearly equivalent to that of an output capacitor 105a in an output circuit 105. The pre-charging transistor 103d pre-charges a side terminal of the coupling capacitor 103c with a reference voltage VR, which is lower than (VDD−Vth).
However, the conventional CMOS image sensor has a problem that a very large layout area is required. That is, the conventional CMOS image sensor requires that the layout area be capable of mounting the sampling transistor 103a, the assistance capacitor 103b, and a reference voltage generating circuit (not shown) for generating the reference voltage VR.